Return UNIX_TIMESTAMP as Python datetime

[judahrand]

What this PR does / why we need it:

Title is pretty self explanatory. However, it should also be noted that before this PR the unix_timestamp_val and unix_timestamp_list_val attributes of Value were never set. So there was a Proto type which was, as far as I can tell, stored incorrectly.

Because this fixes a bug in how UNIX_TIMESTAMP types are represented in the OnlineStore this change could result in a mix of Python types being returned for UNIX_TIMESTAMP features in existing OnlineStores.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Return `UNIX_TIMESTAMP` as Python `datetime`

[codecov-commenter]

Codecov Report

Merging #2244 (dd3f75a) into master (46c4722) will decrease coverage by 25.27%.
The diff coverage is 21.73%.

@@             Coverage Diff             @@
##           master    #2244       +/-   ##
===========================================
- Coverage   85.42%   60.14%   -25.28%     
===========================================
  Files         108      108               
  Lines        9027     9028        +1     
===========================================
- Hits         7711     5430     -2281     
- Misses       1316     3598     +2282     

Flag	Coverage Δ
integrationtests	?
unittests	60.14% <21.73%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...s/integration/registration/test_universal_types.py	35.62% <ø> (-63.13%)	⬇️
sdk/python/feast/type_map.py	42.36% <21.73%> (-26.87%)	⬇️
.../integration/online_store/test_online_retrieval.py	17.39% <0.00%> (-82.61%)	⬇️
.../integration/online_store/test_universal_online.py	15.20% <0.00%> (-82.49%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	20.68% <0.00%> (-79.32%)	⬇️
...fline_store/test_universal_historical_retrieval.py	23.71% <0.00%> (-76.29%)	⬇️
...gration/registration/test_feature_service_apply.py	31.25% <0.00%> (-68.75%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	26.97% <0.00%> (-67.11%)	⬇️
sdk/python/tests/data/data_creator.py	34.78% <0.00%> (-65.22%)	⬇️
sdk/python/tests/integration/e2e/test_usage_e2e.py	35.00% <0.00%> (-65.00%)	⬇️
... and 54 more

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[felixwang9817]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: felixwang9817, judahrand

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